More panic fixes

[Razican]

This PR fixes a couple of panics when working with regular expressions. I also tried to fix a double-borrow in Map but I only got to a bit of a better syntax, without fixing the panic.

[github-actions]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,497	78,497	0
Passed	24,548	24,550	+2
Ignored	15,585	15,585	0
Failed	38,364	38,362	-2
Panics	28	24	-4
Conformance	31.27	31.28	+0.00%

[codecov]

Codecov Report

Merging #1050 (9f066af) into master (c083c85) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1050      +/-   ##
==========================================
- Coverage   58.31%   58.30%   -0.02%     
==========================================
  Files         172      172              
  Lines       12007    12008       +1     
==========================================
- Hits         7002     7001       -1     
- Misses       5005     5007       +2     

Impacted Files	Coverage Δ
boa/src/builtins/string/mod.rs	62.62% <50.00%> (-0.18%)	⬇️
boa/src/builtins/map/mod.rs	71.63% <100.00%> (-0.40%)	⬇️

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[github-actions]

Benchmark for 80ed99f

Click to view benchmark

Test	PR Benchmark	Master Benchmark	%
Arithmetic operations (Execution)	266.3±0.21ns	266.1±0.46ns	+0.08%
Arithmetic operations (Full)	195.7±0.24µs	176.1±0.49µs	+11.13%
Array access (Execution)	5.1±0.01µs	4.6±0.01µs	+10.87%
Array access (Full)	187.8±0.60µs	191.3±0.80µs	-1.83%
Array creation (Execution)	2.4±0.00ms	2.2±0.01ms	+9.09%
Array creation (Full)	2.5±0.00ms	2.5±0.00ms	0.00%
Array pop (Execution)	776.1±4.37µs	709.4±1.27µs	+9.40%
Array pop (Full)	1067.9±1.24µs	1057.8±1.78µs	+0.95%
Boolean Object Access (Execution)	3.7±0.01µs	3.7±0.01µs	0.00%
Boolean Object Access (Full)	182.6±0.43µs	183.7±0.32µs	-0.60%
Clean js (Execution)	503.3±2.28µs	570.7±2.86µs	-11.81%
Clean js (Full)	724.3±6.14µs	726.9±4.22µs	-0.36%
Clean js (Parser)	26.7±0.04µs	26.7±0.07µs	0.00%
Create Realm	401.8±0.55ns	421.2±0.34ns	-4.61%
Dynamic Object Property Access (Execution)	3.8±0.01µs	4.3±0.01µs	-11.63%
Dynamic Object Property Access (Full)	186.3±0.52µs	213.5±0.45µs	-12.74%
Expression (Parser)	5.7±0.00µs	5.7±0.01µs	0.00%
Fibonacci (Execution)	720.1±0.50µs	642.2±0.81µs	+12.13%
Fibonacci (Full)	957.2±2.88µs	840.8±2.14µs	+13.84%
For loop (Execution)	19.2±0.03µs	19.4±0.03µs	-1.03%
For loop (Full)	230.7±0.51µs	233.9±0.44µs	-1.37%
For loop (Parser)	13.1±0.14µs	13.2±0.05µs	-0.76%
Goal Symbols (Parser)	8.9±0.03µs	10.1±0.01µs	-11.88%
Hello World (Parser)	2.3±0.01µs	2.6±0.00µs	-11.54%
Long file (Parser)	611.0±0.90ns	609.7±1.11ns	+0.21%
Mini js (Execution)	510.5±2.58µs	511.0±2.71µs	-0.10%
Mini js (Full)	656.9±2.31µs	655.4±3.70µs	+0.23%
Mini js (Parser)	23.6±0.03µs	23.9±0.55µs	-1.26%
Number Object Access (Execution)	3.3±0.00µs	2.9±0.01µs	+13.79%
Number Object Access (Full)	180.5±0.30µs	206.0±0.28µs	-12.38%
Object Creation (Execution)	3.6±0.01µs	3.3±0.01µs	+9.09%
Object Creation (Full)	181.5±0.20µs	207.5±0.33µs	-12.53%
RegExp (Execution)	7.2±0.03µs	8.1±0.03µs	-11.11%
RegExp (Full)	193.1±0.48µs	193.5±0.65µs	-0.21%
RegExp Literal (Execution)	8.2±0.05µs	9.1±0.02µs	-9.89%
RegExp Literal (Full)	190.0±0.99µs	190.9±0.45µs	-0.47%
RegExp Literal Creation (Execution)	7.2±0.02µs	8.0±0.05µs	-10.00%
RegExp Literal Creation (Full)	185.1±0.46µs	210.9±0.71µs	-12.23%
Static Object Property Access (Execution)	3.4±0.01µs	3.9±0.01µs	-12.82%
Static Object Property Access (Full)	182.9±0.25µs	209.8±0.36µs	-12.82%
String Object Access (Execution)	5.1±0.01µs	5.2±0.02µs	-1.92%
String Object Access (Full)	186.7±0.30µs	186.4±0.58µs	+0.16%
String comparison (Execution)	5.3±0.02µs	4.8±0.02µs	+10.42%
String comparison (Full)	186.0±0.43µs	187.4±0.98µs	-0.75%
String concatenation (Execution)	3.9±0.01µs	4.4±0.01µs	-11.36%
String concatenation (Full)	183.5±0.34µs	182.5±0.61µs	+0.55%
String copy (Execution)	3.3±0.02µs	3.0±0.01µs	+10.00%
String copy (Full)	177.6±0.23µs	180.0±1.28µs	-1.33%
Symbols (Execution)	2.8±0.10µs	2.5±0.03µs	+12.00%
Symbols (Full)	197.3±0.26µs	199.2±0.31µs	-0.95%